A fuel cell is a device which generates electricity or heat energy using an electrochemical reaction between fuel and an oxidizing agent. In general, a fuel cell includes unit cells as a basic structure, each of which contains an electrolyte, two electrodes positioned at both sides of the electrolyte, and two separators provided with channels for flowing fuel such as hydrogen gas or an oxidizing agent such as air, and positioned at both side of the electrodes. When high output is required, a plurality of the unit cells are stacked in series to form a stacked structure, and collects electrical power by power-collecting plates arranged at the both sides of the stack.
A fuel cell has many designs depending on the kind of electrolyte, fuel, oxidizing agent, and the like. Among the designs, a solid polymer fuel cell, which has a solid polymer electrolyte membrane as an electrolyte, uses hydrogen gas as fuel, and air as an oxidizing agent, and a direct methanol fuel cell, which uses hydrogen as fuel which generated from methanol inside thereof, can generate electricity effectively at relatively low operating temperatures such as 100° C. or less.
The separator of these fuel cells is generally a molded plate which is made of a gas-impermeable conductive material containing a conductive material such as graphite and resin, and which has a ribbed structure forming gas channels together with a gas diffusion electrode on the surface thereof. The separator provides supply routes for a reaction gas flowing in the fuel cell by the gas channels, and transfers electricity generated in the fuel cell to the outside. In order to fulfill these functions, the separator is required not only to be made of a material having high conductivity at the surface and in the thickness direction, but also to decrease resistance of the surface in contact with the electrode parts.
To realize this requirement, a separator having a decreased contact resistance to the electrode part and a method for producing the separator are suggested (For example, Patent Documents Nos. 1 and 2). This separator is made by molding a mixture containing a conductive material and a thermoplastic resin or thermosetting resin, and polishing the surfaces of the separator, thereby machining the surface of the separator so as to be adjusted to a specific surface roughness to decrease the contact resistance.
Although, the contact resistance to the electrode part is improved partly in the separator, it is still insufficient. In addition, since the surface is rough, when material having inferior wettability is used, the hydrophilicity of the separator sometimes decreases.
In the fuel cell including the separator, a fuel gas containing hydrogen is supplied to a cathode, and an oxidizing gas containing oxygen is supplied to an anode. When an electrochemical reaction occurs at each electrode, water is generated at the cathode or the anode.
In general, the generated water is evaporated in the oxidizing gas supplied to the anode, and discharged from the fuel cell together with the oxidizing gas. However, when the amount of generated water is large, it is impossible to discharge all generated water only by evaporating in the oxidizing gas. When the generated water, which is not evaporated in the oxidizing agent and remains, forms water droplets around the anode, gas channels are blocked and the flow of the oxidizing gas is prevented around the anode, and this induces a decrease in fuel performance.
Blockage of the gas channels like this may occur not only at the anode but also at the cathode. Although water is not generated at the cathode due to such an electrochemical reaction, water vapor in the fuel gas supplied to the cathode may be concentrated. In general, when the electrochemical reaction proceeds, since protons generated due to a cathode reaction at the cathode side move toward the anode side in the electrolyte membrane while hydrating with a specific number of water molecules, the amount of moisture at the cathode side in the electrolyte membrane is insufficient, and conductivity decreases. In order to prevent such a decrease, the fuel gas supplied to the anode is humidified to compensate for moisture in the electrolyte membrane.
As explained above, water vapor added to the fuel gas may sometimes be concentrated in the gas channels at the time to of starting operation or when the working temperature of the fuel cell decreases and the saturated vapor pressure decreases. In such a case, the gas channels at the cathode side are also blocked preventing the flow of the fuel gas, and this induces a decrease in fuel performance.
As explained above, since protons generated due to the electrochemical reaction at the cathode move toward the anode side while hydrating, water molecules together with the protons reach the anode in addition to the generated water. Due to this, there is a further water exceeds, and the gas channels are easily blocked. This blockage phenomenon is accentuated in a separator for a fuel cell containing a conductive material and resin.
In the past, the attempts have been made to improve discharge efficiency of the generated water is tried to be improved by making the entire surface of the separator for a fuel cell containing a conductive material and resin or the surface of the gas channels hydrophilic. When the material constituting a separator for a fuel cell containing a conductive material and resin is subjected to a hydrophilic treatment, the generated water is not accumulated as water droplets, and is introduced to specific channels. Thereby, it is possible to prevent inhibition of gas diffusion due to the generated water.
The solid polymer fuel cell explained above has a unit cell as a basic unit, which contains a solid polymer membrane as an electrolyte layer, a pair of gas diffusion electrodes sandwiching the solid polymer membrane, and separators sandwiching the gas diffusion electrodes from further outside to separate the fuel gas and oxidizing gas. The solid polymer fuel cell contains a plurality of layered unit cells. In the solid polymer fuel cell, the hydrophilic treatment explained above is not only performed on the separator, but also performed on the gas diffusion electrodes.
Examples of the hydrophilic method for making the separator for a fuel cell having a conductive material and resin hydrophilic include the following methods.
In past days, a method has been suggested, in which a porous water-absorbing carbon material having a pore ratio of 30 to 80% is provided at the inlet or outlet of the gas channels (For example, Patent Document No. 3). However, the separator used in that method has a problem in that hydrophilicity deteriorates as time passes. In addition, the method requires a complex step in which the water-absorbing material is arranged during or after molding, and the method has a problem in the production step.
In addition, as a hydrophilic method, a method has been suggested, in which the surface of the fuel gas channels, and preferably further the surface of the oxidizing gas is covered with a film made of many kinds of hydrophilic resin, hydrophilic organic compounds or hydrophilic inorganic compounds, or coated therewith (For example, Patent Documents Nos. 4 and 5). However, since the separator produced by the method has an insulating film on the surface thereof, this is one factor causing a considerable decrease in conductivity or durability of the fuel cell due to substances being eluted from the film.
Furthermore, as the hydrophilic method, a method has been suggested in which hydrophilic substances such as silicon oxide, aluminum oxide, starch, acrylic acid copolymer resin, polyacrylate, and polyvinyl alcohol, and water-absorbing resin are added to a raw material constituting a conductive separator containing a resin binder to make the separator itself hydrophilic (For example, Patent Document No. 6). However, since the hydrophilic substances or the water-absorbing resin easily absorb water, and many kinds of impurities are eluted from the hydrophilic substances or the water-absorbing resin into water, there is a problem in that performances of the fuel cell having the separator are remarkably inhibited.
In addition, as the hydrophilic method, a method has been suggested in which the surface of the conductive separator made of many kinds of material is subjected to a low-temperature plasma treatment, corona treatment, or ultraviolet irradiation treatment in a hydrophilic gas to make it hydrophilic (For example, Patent Document No. 7). However, the hydrophilic effects decrease as time passes, and there is a case in which the method is required to be carried out in a vacuum in the method. That is, the method has a problem in its process.
Furthermore, a method, in which the surface of the separator is made hydrophilic by carrying out a normal-pressure discharge plasma treatment using a sulfur-containing compound and the like as a treatment gas, has also be suggested (For example, Patent Document No. 8). However, plasma is irradiated to the separator for a fuel cell made of graphite under severe conditions in the method. Therefore, the method has a problem in that graphite on the surface of the separator is oxidized and changes to ash, and this induces a decrease in conductivity or damage of the shape of the molded article, and thus the molded article cannot be used practically as a separator.
[Patent Document No. 1] Japanese Patent Application, First Publication No. 2002-270203
[Patent Document No. 2] Japanese Patent Application, First Publication No. Hei 11-297338
[Patent Document No. 3] Japanese Patent Application, First Publication No. Hei 08-138692
[Patent Document No. 4] Japanese Patent Application, First Publication No. 2003-217608
[Patent Document No. 5] Japanese Patent Application, First Publication No. 2003-297385
[Patent Document No. 6] Japanese Patent Application, First Publication No. Hei 10-3931
[Patent Document No. 7] PCT International Publication No. WO 99/40642 brochure
[Patent Document No. 8] Japanese Patent Application, First Publication No. 2002-25570